Thanks, lodestar, for your explanations and links.
So, the new card has arrived. First thing to clarify - the cooler on the 1GB version is not the same as in 660 or in 650 Ti Boost 2GB. These have in fact 4 heatpipes, while mine has only 1. It's beefy and touches the chip directly, but it's only one. Other than that (and the lack of some plastic shroud at the PCI-E end of the card) the coolers seem the same.
Both fans run at 1100-1140 RPM in idle, in auto mode. This is reported as 35%. They cannot go any lower in manual mode. Still, for now I find them quiet enough. Well, this time of year I keep my windows open, or at least tilted, so even under load the PC stays inaudible. This might change in autumn and later on, however.
Given that the card idles at 30 C deg, 1100 RPM seems awfully high. OTOH, this is very low, given that my ambient is now around 23-24 C. And that I only have one exhaust 120mm fan @ 600 RPM. Still, the card's idle power draw is paltry - my PC idles at 65W. This is the same value as when I use 8400 GS. I bet GB would be perfectly safe to use barely spinning fans with this one. Or even stop them altogether when in 2D.
Just for kicks, I used the Kepler tweaker and NVflash to mod the bios and flash it, and brought down the fan control to 20%. It wouldn't go any lower in the tweaker. Alas, I couldn't hear any change in noise. I did that around midnight, but the ambient was still too loud. I don't really have the time now to rearrange the PC and put it on the desk to actually try and watch if the blades spin any slower.
A nasty side effect of the mod - the fans' RPM readout went crazy. On default bios they would stay pretty much tame, slowly shifting from 1100 to 1200, but after modding the bios they would show values from 1080 up to 1800, changing every few seconds, never settling. I definitely didn't hear these changes (and I certainly would - 1800 is pretty fast), so I gather it's just a quirk. The GB OC Guru showed 30% at the lowest, instead of the original 35% (with 20% being the modded value). Edit - same story when I brought the fan down to 0% in the tweaker. The bios loaded ok, but it still shows 30% and the RPM range goes up / down like stupid. I also observed the temperature of the GPU and it stays 30 C deg, no matter if I use the original bios or the "modded" one.
All in all, I went back to the original bios. Perhaps (despite the crazy fan readings) the change was there but I just simply couldn't hear it in this setup. But i will definitely want to mess with it again
For me personally (and for now) this card rocks - I can't hear it in idle, it's powerful enough for gaming in 1600x1200, and it was v cheap. Some will argue that a 2GB version would perform in FHD better. That's true, but only for some games, in some scenarios and only with really maxed eye candy.
Users report that MSI Twin Frozr cards idle with fans @ around 900 RPM, so I guess for truly hardcore silencers such MSIs would be a better choice.